The present invention relates to a receiver and a method of receiving and, in particular, but not exdusively, to a receiver and method of receiving for use in a wireless cellular telecommunications network.
FIG. 1 illustrate a known wireless telecommunication network 2. The area covered by the network 2 is divided into a number of cells 4. Each cell 4 has associated therewith a base transceiver station 6. Each base transceiver station 6 is arranged to communicate with terminals 8 located in the cell 4 associated with that base transceiver station 6. The terminals 8 may be mobile stations which are able to move between the cells.
Each base transceiver station is, in the GSM standard (Global System for Mobile Communications), arranged to receive N frequency channels out of 125 available channels C1 . . . C125, as illustrated in FIG. 2a. The 125 frequencychannels C1 . . . C125 occupy a bandwidth of 25 MHz. Each frequency channel therefore has a width of 200 KHz. Each channel is divided into frames F one of which is shown in FIG. 2b. Each frame is divided into eight slots S1 . . . S8. The GSM standard is a time division multiple access (TDMA) system and accordingly different mobile stations will be allocated different slots. Thus, the base transceiver station will receive signals from different mobile stations in different time slots in the same frequency channel. N is usually much less than 125.
Reference is made to FIG. 4 which shows part of a known base transceiver station 9 which is arranged to receive N frequency channels at the same time. For clarity, only the receiving part of the base transceiver station 9 is shown. The base transceiver station 9 has an antenna 10 which is arranged to receive signals from mobile stations in the cell served by the base transceiver station 9. The base transceiver station comprises N receivers R1, R2 . . . RN. Thus one receiver is provided for each frequency channel which is to be received by the base station 9. All of the receivers R1-RN are the same and accordingly the components of the first receiver R1 only are shown.
The first receiver R1 comprises a first bandpass filter 12 which is arranged to filter out signals which fall outside the 25 MHz bandwidth in which the available channels are located. The filtered output is input to a first low noise amplifier 14 which amplifies the received signals. The amplified signal is then passed through a second bandpass filter 16 which filters out any noise, such as harmonics or the like introduced by the first amplifier 14. The output of the second bandpass filter 16 is connected to a mixer 18 which receives a second input from a local oscillator 20. The frequency of the output of the local oscillator 20 will depend on the frequency of the channel allocated to the particular receiver. The output of the second bandpass filter 16 is mixed with the output of the local oscillator 20 to provide a signal at an intermediate frequency IF, which is less than the radio frequency at which the signals are received. The intermediate frequency IF output by the mixer 18 of each receiver will be the same for all receivers and may, for example, be 180 MHz. For example, if the channel allocated to a given receiver has a frequency of 880 MHz then the local oscillator 20 of that receiver will be tuned to 700 MHz. On the other hand, if the channel allocated to a given receiver has a frequency of 900 MHz, then the local oscillator will be tuned to a frequency of 720 MHz.
The output of the mixer 18 is input to a third bandpass fitter 22 which filters out any noise introduced by the mixer 18. The output of the third bandpass filter 22 is amplified by a second amplifier 24 and output to a surface acoustic wave (SAW) filter 26 or another filter of an appropriate type. The surface acoustic wave filter 26 filters out all signals except that of the channel allocated to that particular receiver. In other words, all the channels received by the antenna 10 with the exception of the channel allocated to the receiver will be filtered out by the surface acoustic wave filter 26. The output of the 30 surface acoustic wave filter 26 is connected to an automatic gain control unit 28 which alters the gain of the signal so that it falls within the dynamic range of an analogue to digital converter 30.
One problem with the known architecture is that it is necessary to provide a receiver for each frequency.
With the known networks, the base transceiver station is required to receive signals from mobile stations 8 which are very close to the base transceiver station as well as from mobile stations 8 which are on the edge of a cell. Accordingly, the strength of the signals received by the base transceiver station will vary a great deal, depending on the distance between the mobile station and the base station. In this regard, reference is made to FIG. 2c which shows the signal received from eight mobile stations, on eight different channels at the same time. As can be seen, the signal from the fourth mobile station MS4 is very much stronger than the signal from the second mobile station MS2. The variation In the amplitude of the received signals gives rise to difficulties in the receiver.
If a single receiver were to be used with signals from more than one channel, amplifiers would have to amplify all of the signals received by the same amount at a given time including the signals with the larger amplitude and those of a smaller amplitude. The larger signals may therefore fall outside the dynamic range of the analogue to digital converter which may cause the analogue to digital converter to become saturated which lead to distortion. Typically, the distortion will take the form of intermodulation distortion which generates intermodulation product signals. This interference can interfere with signals received on other frequencies. If a lower amplification is used, this may result in the smaller signals being lost or swamped by background noise.
It is therefore an aim of some embodiments of the present invention to reduce or at least mitigate these problems.
According to one aspect of the present invention, there is provided a receiver for receiving a plurality of different signals at the same time, said receiver comprising means for identifying at least one strongest signal of said plurality of different signals; and means for attenuating said at least one strongest signal with respect to the other of said plurality of signals wherein the said attenuating means comprises hybrid means.
Thus, as the, at least one, strongest signal is identified and attenuated, the range of amplitudes of the signals which are provided for subsequent processing is reduced. This can avoid the problem of signals falling outside the dynamic range of, for example an analogue to digital converter.
Preferably, the attenuating means are arranged to allow signals which are not to be attenuated to pass therethrough substantially without change. This allows a single path to be provided for all signals with only the at least one strongest signal being attenuated, the other signals remaining substantially unchanged.
The plurality of different signals are preferably at different frequencies.
The hybrid means may be coupled to at least one tunable filter means, said at least one tunable filter means being tuned to a frequency of the signal to be attenuated. The tunable filter can take any suitable form and may be mechanical or electric. The frequency to which the at least one tunable filter is tuned may be controlled by the output of the identifying means. Preferably, at least one resistive load is connected to the hybrid means. Variation in the value of the resistive load may allow the degree of attenuation provided by the attenuation means to be controlled. The degree of attenuation provided may also be controlled by the tunable filter means.
The identifying means is preferably arranged to control the operation of the attenuating means. Thus, the identifying means preferably provide control signals which are used to control the attenuating means. For example, the identifying means may control the frequency to which the at least one tunable filter means, if provided, is tuned.
Preferably, a plurality of stronger signals are identified. The attenuating means may therefore attenuate a plurality of the strongest signals. In one embodiment of the present Invention, the two strongest signals are identified. However, it should be appreciated that more than two of the strongest signals can be identified.
Preferably, the identifying means is arranged to determine if the magnitude of said at least one strongest signal exceeds a threshold and the attenuating means is arranged to attenuate the at least one strongest signal only if the at least one strongest signal exceeds the threshold. Thus, signals which are not too large, despite being the strongest signals, are not unnecessarily attenuated.
Preferably, splitter means are arranged to provide two sets of signals from the plurality of different signals, each set containing all of the plurality of different signals. One set of the signals is preferably provided to the Identifying means whilst the other set of signals is provided to the attenuating means. The frequency of the set of signals provided to the identifying means may be reduced before the signals are provided thereto. This may be advantageous if the received signals are at a radio frequency and the identifying means requires the frequency of the signals to be at a lower frequency to obtain the required information.
A base station incorporating a receiver as described hereinbefore is preferably provided.
According to a second aspect of the present invention, there is provided a method of processing a plurality of different signals received at the same time, said method comprising the steps of identifying at least one strongest signal of said plurality of different signals; and attenuating said at least one strongest signal with respect to the other of said plurality of signals wherein attenuating method step uses hybrid means.